Figure 1. Map of desalination facilities on Eleuthera
Commercial Approach & System Design
The system will be designed, built, installed, owned, operated and maintained under a commercial model called build, own and operate. Under this commercial model, Aqua Design guarantees the quality and quantity of product water and provides all capital necessary to install and operate the plant.
The system starts with raw seawater wells approximately 150 ft deep. Well water is pumped through two stages of filtration (cartridge and multi-media), which provide water with low suspended solids content. Filtered water is sent to a single-pass reverse osmosis (RO) plant, which removes 95% to 98% of the dissolved solids.
The RO filtration process prevents the passage of most dissolved solids. The water is divided into two streams —permeate containing water and a small fraction of dissolved solids and concentrate; and reject brine containing water and most of the dissolved solids present in the feed. Typically, approximately 50% of the inlet water is recovered as product water. Reject brine contains about two times the concentration of sea water.
The RO process uses high-pressure pumps to force water from the feed to the permeate side of the membranes. The high pressure is required to overcome the osmotic pressure of the membranes. Energy recovery devices are used to recover some of the energy in the recovered water pressure, boosting the incoming water pressure and lowering the system’s overall energy consumption.
Product water from the RO process is adjusted for pH, and calcium carbonate is added in a remineralization bed. This product water then is sent to storage for distribution to the community by W&SC. Backup power generation will be used when power supply is interrupted.
Dissolved solids removed by the process are injected into a brine disposal well, approximately 250 ft deep.
This process is commonly used throughout the Caribbean and is utilized at the other Aqua Design facilities on Eleuthera. W&SC has used the build, own and operate commercial approach on several islands of the Bahamas, as it allows W&SC to increase production while conserving capital and relying on the design, logistics and operating expertise of companies such as Aqua Design.
